The one on the right is the repro cover I bought years ago and the one on the left is one I got at a swap meet that was the right date for my engine. However, the timing tab is welded at the top and not the bottom and it doesn't seem to be for an 8" balancer even though it says it is. What is the one on the left for?

7"---327 ? could be your swap meet one had the tab broke off and was "fixed" incorrect. Mine was broke off many years ago and replaced with an aftermarket bolt on tab. (not me) with timing tape on the balancer.

Measured from the bottom of the seal hole to the tab is 6" on the left one. 6 1/2" on the right. I bought it because it said it was an 8' balancer and had my date, but looking through all the info I could find in books and here all the 8" ones had the tab mounted below the timing marks as the one on the right has. Must just be the plain old 7" one. If anyone needs the 5-68 date let me know.

I believe the "swap meet" cover is actually made for the (smaller diameter) base engine dampener. The cover itself is the same either way but the timing tab would need to be welded farther away (and flipped around) to fit the bigger diameter HP damper. The spot welds are in the same place either way but the welds would be "under" the tab for the big damper and "above" the damper for the smaller base engine damper. The 90° bend in the timing tab would be in the opposite direction if you look at the pix.